The present invention relates to a liquid crystal apparatus used to display several kinds of characters, marks, images or the like, and more particularly to a highly reliable liquid crystal apparatus in which the lead electrodes in a liquid crystal device are securely connected to the lead electrodes of a drive circuit board with less resistance.
Recently, liquid crystal apparatus have been designed for television display, graphic display and the like. Therefore, it has become important to develop liquid crystal apparatus having a large capacity and a high density. Such liquid crystal apparatus have drive circuit boards which are connected to a liquid crystal device.
In the conventional liquid crystal apparatus, in order to connect a liquid crystal device with a drive circuit board, the following method is adopted. An electrically conductive rubber connector is disposed between each lead electrode of the liquid crystal device and the associated lead of the drive circuit board. Both are retained, as necessary, from the outside by a retaining mechanism, such as a spring member, to electrically connect both lead electrodes.
The electrically conductive rubber connector is formed of several conductive layers, each of which is made of an insulating material such as silicone rubber or the like contains electrical conductive powder such as carbon powder, metal powder or the like. Each conductive layer is formed so as to have a thickness which is less than the width of a lead electrode and also less than the width of a space between lead electrodes. This conductive layer is produced by using a silicone rubber in a normally liquid form containing electrical powder dispersed therein, which is disposed between magnets to collect the electrical conductive material around magnetic poles, and is hardened in this condition.
With the electrically conductive rubber connector, there is a problem when the electrical conductivity of the boundary between each adjacent conductive layer and insulator in the electrically conductive rubber connector gradually varies. This problem creates difficulty in thinning the connector and causes low accuracy. Therefore, it has been difficult to connect several lead electrodes with a high density.
Further, since retaining mechanisms are generally required, there has been difficulty in thinning and miniaturizing of the products.
In order to address these problems, there have been investigations of a method of connecting lead electrodes in which an anisotropic conductive layer, made of organic resin containing electrically conductive particles having substantially equal particle sizes, is disposed between the lead electrodes of a liquid crystal device and a drive circuit board. The layer is subjected to thermocompression bonding so that the electrically conductive particles between the lead electrodes of the liquid crystal device and the drive circuit board are in presscontact with both lead electrodes.
However, the lead electrodes of a common electrode for a liquid crystal device and the associated lead electrodes of the drive circuit board have a relatively wide width, such as, for example, about 2 mm in conventional liquid crystal apparatus. Wide widths are used so that the electrical resistance is reduced.
Unfortunately, with the above method, no relieving places for organic resin exist in the wide lead electrodes of the common electrode and the associated lead electrodes of the circuit board. Therefore, a large amount of organic resin exists between both electrodes, and the thickness of the anisotropic electrical conductive layer is unaltered. Accordingly, the contact of the electrically conductive particles to the lead electrodes is inferior, resulting in a large contact resistance. In the worst case, electrical connection between the electrodes cannot be obtained.
For example, in the case of an active matrix type liquid crystal apparatus in which electrical connection is made by the above mentioned method, an electrical resistance of about 0.2 to 0.5 .OMEGA./lead exists between the lead electrodes of the liquid crystal device other than the lead electrode and the associated lead electrodes of the circuit board, while an electrical resistance of about 1.0 to 1.5 .OMEGA./lead exists between the lead electrodes of the common electrode and the associated lead electrodes of the circuit board. Further, in a test of reliability, under an atmosphere in which the temperature is 80.degree. C. and the humidity is 95% and a liquid crystal apparatus is left for 10 to 100 hours, the former resistance is changed to about 20 .OMEGA./lead, while the latter resistance is changed to about 200 .OMEGA./lead.
Further, as the testing time is made longer, the former resistance is slightly increased while the latter resistance is abruptly increased to a value in a range form several k.OMEGA. to several hundred k.OMEGA., and in the worst case, an open circuit is effected.